There have been proposed various types of power transmission mechanisms applying many types of gears such as a planetary gear system, a bevel gear system, a worm with worm wheel system and the like. These conventional mechanisms, however, have generally complicated structures and need to make use of gears which increases the cost of manufacturing. Further, the complicated structure increases the size of the mechanism which also increases the cost, and the cost of the mechanism is a large part of the cost of the entire machine into which it is incorporated.
Moreover, in mechanisms which make use of the bevel gear system or the worm with worm wheel system, an axis of the input shaft is crossed with an axis of the output shaft at either right angles or at some other predetermined angle. The crossing of the shafts increases the difficulty of designing the machine to be compact. Accordingly, it has been desired to provide a compact power transmission mechanism which has an enhanced power increase and decrease ratio such that an axis of the input shaft and an axis of the output shaft are arranged in parallel or in alignment.